Substrains of S. mansoni selected for varying infectivity to vector snails have been tested for their ability to induce immunity in mice. Two strains derived from the same patient and designated PRT-3 and PRC-3 induced markedly different degrees of resistance to reinfection after a bisexual first infection. The F-1 cross between these strains also produced high resistance, comparable to that induced by the "immunogenic" PRT-3 strain. These two schistosome strains have been selected for two generations on the basis of their ability to induce resistance, without evident success thus far. Unisexual infections with some schistosome strains have induced considerable resistance to second infections, but the relative importance of worm strain and mouse strain in the induction of resistance are unknown.